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No 'shaky start' for Opel/Vauxhall’s new Corsa triple

2014-12-03
Stuart Birch

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Thorsten Kniesa likes a quiet life, and as a senior member of the team that created Opel Group’s (Opel and Vauxhall) three-cylinder Ecotec gasoline engine, he has helped ensure that buyers of the just launched Corsa hatchback can also have one.

If any compact car demonstrates convincingly the potential of a 1.0-L triple linked to a six-speed manual transmission as a downsizing powertrain solution, it is the little fourth-generation Corsa. And probably the most significant single mechanical contributor to that peaceful traveling is the use of a counter rotating balance shaft.

“It is a highlight for a three-cylinder, high-volume production engine, and it is there simply for reduced NVH (noise, vibration and harshness) reasons. Yes, it adds a little cost and weight, but it is a very good investment,” said Kniesa, Assistant Chief Engineer for three-cylinder engines. “Without it, the engine would have different vibrations, and they would be felt by the occupants. This would be particularly noticeable during start-stop operation. Without it, there would be what we call a ‘shaky start,’ reminding the car’s occupants that there is one cylinder less than normal. They don’t want to know!”

The Opel Group (the nomenclature replaces GM Europe and includes all General Motors activities in Europe including Russia) triple, previously detailed by http://articles.sae.org/12970/, is also available in the smaller Opel/Vauxhall Adam, but with Corsa it is now entering the big time in volume terms.

Both engine and car were wholly developed in Germany at Opel’s Rüsselsheim Technical Center. The engine follows Ford’s highly successful and impressive three-cylinder EcoBoost, a power unit that has a distinctively sporty note when extended, a feature that many buyers savor. However, Kniesa says that such a sporty aural signature was not what Opel Group aimed to achieve; exceptionally low NVH was the target, and the balance shaft was part of that.

Ford says that the EcoBoost’s flywheel has been deliberately unbalanced to deliver required levels of smooth running, without the necessity for a balance shaft, which would reduce engine efficiency. Volkswagen also does not use a shaft on its 1.0-L gasoline triple. The company claims that reciprocating parts are so light that a shaft is not necessary, although its triple 1.4-L diesel does have a shaft.

Andy Delicata, Ford of Europe Manager, Powertrain NVH, explained at the engine’s announcement that the traditional way of reducing shaking forces in small-displacement engines is to install a counter-rotating balance shaft to cancel out most vibrations. But the problem with a balance shaft, he said, is that it is heavy, expensive, and it reduces fuel economy.

So his 1.0-L NVH engineering team attacked the problem by focusing on two areas: the engine’s front pulley and rear flywheel, and the mounting system that connects the powertrain with the car’s body. He said that the pulley and flywheel are unbalanced with weights that are placed precisely to counteract the natural shaking forces of the engine and drive the energy in a less sensitive direction. The engine mounts are designed to decouple as well as absorb the engine’s shaking forces.

But Kniesa said Opel Group is confident that the benefits of a shaft overshadow the downside of its use. The shaft adds around 2% to engine unit weight—the dressed Ecotec unit has a mass of 106 kg (234 lb)—and up to 5% to unit cost, both figures with which Opel and Vauxhall are comfortable.

Significant though the balance shaft is, it is just part of an integrated assault on the new Corsa’s NVH. “For the Ecotec program, there were three engineering tasks for a three-cylinder, continuously variable valve timing engine: best refinement, together with very good fuel efficiency and performance,” said Kniesa.

How to benchmark refinement was an interesting challenge, he said, because 42 months ago when the engine and new Corsa programs started, there were very few triples in production: “At the time, three-cylinder engines were generally regarded as being an affordable entry-level for small cars, without the stress on refinement.”

GM had a 40-kW (54-hp) non-turbo entry-level gasoline triple in 1997, which was not the most impressive power unit of its day.

“But now for the new Corsa, we are introducing our three-cylinder with a turbocharger, direct fuel injection–and the balance shaft. So we set our own standards and successfully compared it to our four-cylinder engines for both NVH and performance. Using direct injection and a high compression ratio (10.5:1), you get closer to the sound of a diesel. The high-pressure fuel system delivers 200-bar pressure, so we decoupled it from the cylinder head; there are no direct metal-to-metal components. We did the same for the heat shield, which can act as a sounding board; it was decoupled and stiffened.”

But getting NVH down to really low levels called for far more, including great attention to fundamental design and a very stiff architecture for the all-alloy engine, bed plate, and two-piece oil pan: “All components had to be optimized for the required sound level, which means shielding, decoupling, and encapsulating.”

The engine’s behavior also contributes to the perception of quality and the reality of low NVH. Available initially at two power levels, 66 and 85 kW (89 and 114 hp), both versions achieve maximum torque of 166 N·m (122 lb·ft) at a low (for a gasoline engine) 1800 rpm, reducing the necessity to take it to (noisier) high revs to extract good performance. The six-speed gearbox through which it drives has been newly designed to minimize friction, is compact, and has a mass of 40 kg (88 lb), making it one of the lightest six-speeders on the market, claims Opel Group.

While the three-cylinder engine represents the main mechanical interest of the new Corsa, other engines include an improved 1.3-L CDTi (diesel) with CO2 emissions of 85 g/km and 1.2-L and 1.4-L gasoline units including a turbo but designed for economy rather than performance. The latest Corsa range also has a new chassis and sits 5 mm (0.2 in) lower than the outgoing model.

Virtually everything forward of the A-pillars is new or upgraded. “The chassis does not inherit a single component from the previous model,” said Michael Abelson, GM’s Vice President Vehicle Engineering in Europe. “We turned every conceivable setscrew to achieve a better drive experience with new levels of comfort, precision, and pleasure.”

MacPherson struts at the front receive a new geometry and new knuckles to improve roll behavior and steering response. Rear suspension includes a compound crank with coil springs. The steering is electric with a City Mode for low-speed maneuvering. A new, reinforced subframe increases torsional rigidity.

GM used Millbrook Proving Ground in the UK as part of its chassis development program together with British public roads, notorious for often poor surfacing and, on minor roads, a plethora of potholes. A prototype was taken to Britain for several weeks of testing 12 months before the car was launched.

The new Corsa’s curb weight is similar to that of the previous generation. The 85-kW triple three-door weighs from 1045 kg (2304 lb) plus 75 kg (165 lb) for the driver. The five-door model starts at 1088 kg (2399 lb) plus 75 kg for the driver depending on specification, about the same as the previous generation. Length is 4020 mm (158.3 in), width 1944 mm (76.5 in), and height 1479 mm (58.2 in).

The 85-kW Corsa reaches 100 km/h (62 mph) from rest in 10.4 s and has a 195-km/h (121-mph) top speed. The lower powered version achieves an official combined fuel consumption of 4.3 L/100 km and CO2 emissions of 100 g/km.

The Corsa’s design was overseen by British Design Director Mark Adams, who has been responsible for giving Opel/Vauxhall models a distinctive brand signature including the “blade” running across the lower door sections.

Like Ford with its triple, Opel Group will further develop the Ecotec with more powerful options in prospect, possibly with an electric compressor configuration to improve turbocharger efficiency. The six-speed manual gearbox at present has a 30 N·m (22 lb·ft) margin before reaching its 200-N·m (148-lb·ft) limit, so a significant increase in torque is also likely.